I recently bought a 3/8" ar500 steel target specifically for shooting surplus steel core 7.62x54 @100-150yds. From my experience the only way that you even significantly cratered a 1/2" plate @200yds is if the range is lying to you and it is NOT ar500 steel or you accidentally bought actual armor piercing ammo and not the regular mild steel core com-bloc surplus.

My guess would be that somebody else either severely cratered the target before you shot with something significantly more powerful, making your rounds penetrate the last bit of steel instead of leaving very small craters themselves; OR somebody used actual armor piercing ammo sometime shortly before or after you and didn't get caught, leaving the range thinking you did it with your MILD steel core surplus.